http: don't write empty data on req/res end()

When calling OutgoingMessage.end() with empty data argument, avoid
writing to the socket unless there's still pending data to be sent.
Fixes: #41062
PR-URL: #41116
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: santigimeno

lib/_http_outgoing.js

@@ -878,9 +878,10 @@ OutgoingMessage.prototype.end = function end(chunk, encoding, callback) {
 
 if(this._hasBody&&this.chunkedEncoding){
 this._send('0\r\n'+this._trailer+'\r\n','latin1',finish);
-}else{
-// Force a flush, HACK.
+}elseif(!this._headerSent||this.writableLength||chunk){
 this._send('','latin1',finish);
+}else{
+process.nextTick(finish);
 }
 
 if(this.socket){

test/parallel/test-http-sync-write-error-during-continue.js

@@ -42,11 +42,11 @@ Connection: close
 // parser.finish() to be called while we are here in the 'continue'
 // callback, which is inside a parser.execute() call.
 
-assert.strictEqual(chunk.length,0);
+assert.strictEqual(chunk.length,4);
 clientSide.destroy(newError('sometimes the code just doesn’t work'),cb);
 });
 req.on('error',common.mustCall());
-req.end();
+req.end('data');
 
 sync=false;
 }));